This invention relates to the shielding of radiation emitted by nuclear reactor components during the removal thereof from the nuclear reactor, and particularly, to a method and apparatus for absorbing radiation when such components are removed from the reactor during maintenance and refueling operations.
A nuclear reactor includes a number of components within the reactor shell, such as fuel rods, extension or lifter rods supported by a grid, etc. From time-to-time, it is necessary to remove some or all of such components from the reactor for maintenance and/or refueling purposes. As is known, such components are radioactive, and it is both necessary and desirable to protect maintenance personnel with respect to radiation energy emitted by said components. For example, the federal standard for exposure of an individual to such radiation is 3000 millirems in a quarter of a year, with 5000 millirems as a maximum yearly dose, but some nuclear reactor operating companies have a requirement that the exposure not exceed 1250 millirems in a quarter of a year.
It is known to use water as a shield in nuclear reactor installations during the opening of the reactor and the removal of components therefrom. For example, it is known to provide a tank (refueling pool) with concrete walls around and spaced from the reactor but having its walls extending above the top of the reactor and to fill the tank close to the top thereof with water with additives, such as boric acid, while the reactor is open to absorb most of the radiation from the reactor and its components during the time that it is open. A crane is provided above the tank for the removal of the reactor cover and such components.
It is sometimes found that in some of the older nuclear reactor installations having such a tank, the depth of the tank is such that when the tank is substantially filled with water, the water does not completely cover the reactor components after they are removed from the reactor and placed in the water resting either directly on the bottom of the tank or on a stand resting on the bottom of the tank.
For example, one known installation includes a component known as the "upper internals", which consists of a plurality of lifter rods, e.g. 53 rods, detachably connected to the control rods during the operation of the reactor to control the nuclear chain reaction. The height of such upper internals is such that when they are removed from the reactor and placed in the water-filled tank, they are not completely covered by the water. Thus, the upper end of the upper internals may project to a greater or lesser extent above the upper surface of the water, which means in some cases that as much as 16 inches of the upper ends of such internals will not be shielded. The radiation level of the exposed portion of the upper internals may be of the order of 100-200 millirems per hour which means that personnel exposed to such radiation may reach their radiation quota within only about six hours of exposure. Of course, if the normal refueling time of several days applies, this could mean serious reduction in the number of skilled supervisors or foremen who are continuously available.
Therefore, it is desirable to use some type of shielding between the maintenance personnel and the parts of the upper internals protruding from the pool. In the past, radiation exposure of workers has been reduced by the use of lead blankets on the conventional manipulator crane which is positioned adjacent to the exposed tops of the upper internals as they rest in their stored position in the refueling pool. However, due to weight restrictions imposed by crane manufacturers, crane indexing problems caused by the weight of the lead blanket, and the difficulties encountered in attempting to produce effective shielding by placing lead blankets over such large equipment as a crane, the use of lead blankets for shielding is not entirely satisfactory.
Because of other structure in the reactor containment vessel, such as cranes and their equipment, it is practically impossible to increase the depth of the tank, and hence, the depth of the water, and it would be expensive to increase the depth of the tank, bearing in mind that the tank may have concrete walls two feet thick and spaced about 20 feet apart, and the walls may be over 100 feet in length. Furthermore, an increase in the depth of the entire tank would be unnecessary.
One object of the invention is to provide complete shielding for a radioactive reactor component, such as the upper internals, which is removed from a reactor and which has a height greater than the depth of the water in the water filled refueling tank when the latter is filled to capacity.